UV image 'path not writable'
One of the reasons that you can not use Alt+S is that the generated image does not have a link to a file on the hard drive. Instead use F3 in the UV/Image Editor window to save as a new file. When the image is linked to a file on the hard drive you can use Alt+S shortcut to override the existing file.

Another reason can be, that the image file is open in another program, that is preventing from the file to be overwritten.